Vector store migration to PostgreSQL

[Enniwhere]

Description:
We migrate the vector store as a separate task.

Acceptance criteria:

• Update create_vector_index logic in vector_store.py
• Update search_vector_index logic in vector_store.py
• Update add_chunks_to_vector_index in vector store
• Update other vector operations (add single article, remove article, add precomputed embeddings, update_vector_index)
• Update metadata table

Technical details:
Optional technical details for context.